1. Field of the Invention
The disclosed subject matter is in the field of laser light control and more specifically in the field of controlling the wavelength of a laser light source as may be used in semiconductor photolithography processes.
2. Related Art
Photolithography is a commonly used process in the semiconductor industry. Modern photolithography typically uses a laser light source to provide very narrow band light pulses which illuminate a mask thus exposing photo-resistive material on silicon wafers. However, advances in semiconductor device parameters have put increasing demands on the performance characteristics of the laser light sources used. Improvements in precision and speed of operation are increasingly needed.
Referring now to FIG. 1, a block diagram of a laser system 100 as may be used in a modern photolithography process can be seen. The source of the light in laser system 100 is a master oscillator (MO) chamber 120.
As is known, when MO chamber 120 fires the resulting light enters Line Narrowing Module (LNM) 110 where it shines through a prism (actually several prisms) and onto a grating within LNM 110. This acts as a light wavelength selector in that changing the position of the prism in LNM 110 changes the wavelength of the laser light. This changed wavelength laser light passes back through MO chamber 120 to an output coupler (OC) 130 and then on to, for example, a stepper-scanner device (not shown) which is responsible for handling and exposing the semiconductor wafers.
Output coupler 130 also passes the laser light output from MO chamber 120 to a Line-center Analysis Module (LAM) 170. LAM 170 is a wavelength sampler which measures the wavelength of the light output from MO chamber 120. The laser light output measurement is then passed from LAM 170 to control computer 160.
Control computer 160 uses the light output measurement to determine what changes need to be made to reposition the prism in LNM 110 to achieve the desired laser light output wavelength for the next laser firing event. The position of the prism in LNM 110 is controlled by a voltage applied to a piezoelectronic transducer (PZT) 140 connected to the prism in LNM 110. Control computer 160 therefore determines what voltage should be applied to PZT 140 to achieve the new desired prism position.
Control computer 160 outputs to PZT drive electronics 150 a digital signal indicating the desired voltage change to be made to the prism in LNM 110. PZT drive electronics comprise a digital-to-analog converter (DAC) that converts the control computer 160 digital signal to an analog voltage signal and an analog low pass filter which reduces high-frequency electrical noise and amplifies the analog DAC voltage signal. This analog voltage signal is then passed from PZT drive electronics 150 to PZT 140 which repositions the prism in LNM 110 which in turn causes a change in the wavelength of the light output from MO chamber 120 through output coupler 130 at the next laser firing event.
This process continues as the stepper-scanner requests a further sequence or burst of light pulses from the laser system at a specified pulse rate, start time and wavelength.
Various challenges have arisen as the photolithography process has advanced over the years. For example, the reduction in semiconductor feature size has caused a reduction in the desired wavelength of the laser light source to maintain the desired focus. Reducing the wavelength requires ever greater precision in the output laser light.
A further challenge is created by pulse periods which vary from 167 to 600 microseconds. At high pulse rates, very little time is available for LAM 170 to take a measurement, pass it to control computer 160, for control computer 160 to calculate a new voltage value for the prism in LNM 110, pass it to PZT drive electronics 150, for PZT drive electronics 150 to analog convert and filter the new voltage value, pass it to PZT 140 and for PZT 140 to change the prism position in order to change the wavelength of light from MO chamber 120 before the next laser light pulse is to occur.
Referring now to FIG. 2, a timing diagram of the above sequence can be seen. Two laser firing events from a sequence of such pulses in a burst are shown, the first indicated as occurring at time t0 and the second indicated as occurring at time t3. After the first laser firing event at time to, the resulting output wavelength is measured by LAM 170 (referring again to FIG. 1). The delay in LAM 170 processing to measure the output wavelength and provide it to control computer 160 is shown in the figure as the LAM delay from time to t0 time t1. The time at which a new control signal is then applied to the prism in LNM 110 is shown as time t2. The delay between time t1 and t2 is the time taken by control computer 160 to calculate a new voltage and have that new voltage be propagated through PZT drive electronics 150 to PZT 140 to reposition the prism before the laser fires at t3.
The LAM 170 measurement delay is fairly fixed in that it depends upon LAM 170 processing time and transmission time from LAM 170 to control computer 160. However, the laser firing rate (the time between t0 and t3) is dictated by the stepper-scanner. Ultimately, with a faster laser firing rate, the LAM 170 measuring delay could potentially be long enough such that a subsequent laser firing event might occur before a new control signal is applied. If that were to occur then there would be a performance hit because the new control signal would then be based on a measurement from an earlier laser firing event rather than on the most recent laser firing event (i.e., it would be one pulse behind). What is needed, therefore, is an improved laser control which can operate with ever-increasing laser firing rates.
A further problem with known approaches to laser control are various disturbances in laser system 100 which make it more difficult to precisely determine how to position the prism in LNM 110. What is additionally needed, therefore, is an improved laser control which can address the various disturbances that occur in a laser system.